The present invention relates to article handling, and more particularly to article holders that handle semiconductor wafers and other articles.
During fabrication of integrated circuits from semiconductor wafers, the wafers are stored and transported in cassettes. FIG. 1 illustrates a cassette 110 holding a number of wafers 120. Each wafer rests on shoulders 130A, 130B formed on cylindrical cassette walls 140A, 140B. A robot (not shown) transports the wafers between the cassette and wafer processing equipment. An article holder (“end effector”) mounted on a robot arm holds the wafers by vacuum or electrostatic forces, or using a gas vortex, or by other means.
When a wafer is thin (below 100 μm), the wafer is difficult to store in a cassette. The wafer sags under its own weight, and can fall off the cassette shoulders 130A, 130B. A thin wafer can also be warped due to the internal stresses caused by the presence of different materials (semiconductor materials, metals, dielectrics) which constitute the wafer circuitry. Therefore, thin wafers have been stored in horizontal wafer shipment containers (“pods”), such as pod 210 in FIG. 2. Wafers 120 are stacked in the pod on top of each other, with paper or plastic inserts 220 separating the wafers. Foam 230 is provided at the top and bottom of the stack (the top piece of foam is not shown). The foam and the paper or plastic inserts have roughly the same shape as the wafers, and most or all of the bottom surface of each wafer is in physical contact with the paper inserts above and below the wafer. The wafer circuitry is protected however. A typical fabrication process starts with fairly thick wafers (e.g. 600 to 800 μm for silicon wafers) which are stored in a cassette. When the wafer circuitry has been fabricated and a protective passivation layer has been formed on the circuitry side of the wafer, the wafer is thinned down from the other side to a final thickness (which may be 100 μm or less), and is stored in a pod. At this stage, the wafer circuitry is protected by the passivation layer, so the physical contact between the wafer 120 and the paper or plastic 220 is acceptable.
It is desirable to provide wafer handling techniques suitable for storage of wafers in pods.